1. Field of the Invention
The present invention relates to a semiconductor integrated circuit, and more specifically, to a semiconductor integrated circuit controlling a variable capacitance capacitor using Micro Electro Mechanical Systems (MEMS) (hereinafter, referred to as a “MEMS-type variable capacitance capacitor”) and a method of controlling that MEMS-type variable capacitance capacitor.
2. Description of the Related Art
MEMS-type variable capacitance capacitors form their electrodes at relatively-movable portions with a MEMS structure and cause a physical change to the distance between two electrodes by deformation of the movable portions, which allows change in capacitance between the electrodes (see, for example, Japanese Patent Laid-Open No. 2002-36197). For example, in addition to capacitor's electrodes (primary electrodes), auxiliary electrodes are also provided for deformation and capacitance change. The auxiliary electrodes may increase electrostatic capacitance of the capacitor by applying a voltage (deformation voltage Vact) between the auxiliary electrodes and decreasing the distance between the capacitor's electrodes by the Coulomb force. Conversely, when the voltage application between the auxiliary electrodes stops, elastic force acts thereon to restore the original form against the deformation, the capacitor's electrodes return to the original positions, and the electrostatic capacitance of the capacitor is reduced to the original value.
When the electrostatic capacitance of the capacitor is required to be kept constant after decreasing the distance between the electrodes, it is necessary to keep the distance between the capacitor's electrodes constant while continually applying a constant voltage (hold voltage Vhold) between the auxiliary electrodes. In addition, the hold voltage (Vhold) is lower than a deformation voltage (Vact) for use in decreasing the distance between the electrodes.
It is desirable that the hold voltage Vhold is set as low as possible in order to reduce the amount of charges trapped into the insulator between the auxiliary electrodes, to improve the reliability of MEMS-type variable capacitance capacitors, and to reduce the power consumption.